I'm fairly sure that your plant is a White Campion .... unfortunately I can't count the styles (Night-flowering should have 3, white 5) but from the colour of the flower I'd be pretty sure. Its a lovely pure white whereas S. noctiflora has a rather grubby faint yellow-green tinge often with a slightly pinkish suffusion to the rear of the petals.

On parts of the Kent Coast (eg. Dover cliffs) you'll also find S. nutans the Nottingham Catchfly which also has a rather off-white flowerbut is distinctly perennial.